The key allotment is really a little cryptic. A better statement is that the EWS can support up to 10 transponder chips. If you have a device such as an AK90, then you can reuse transponders spots (i.e. one of the 10) versus just deactivating them and configuring another. So, it isn't as though if you lost 10 keys you'd be up the proverbial creek. That is what many threads on the net imply and it is NOT true if you are using an AK90 type device. Other programming tools may be different and might not let you reuse spots.
Now for the transponder chips themselves.... Yes, the are OTP (one time programming) type devices. HOWEVER, the guys you have in your key now were clearly paired with the old EWS. I really don't know everything that is programmed into the transponder chip. If the differentiating piece of data is the VIN (which is in the chip), then your old transponder chips may well work. I just don't know... sorry! It would be nice if you could use them, though, because that means you don't need to buy anything else. And getting those chips was a major PITA! Look in this thread for a cross reference to the AS30 chips that were unobtainium for me. The replacements (AS32? don't recall) were easier to find and worked fine in the end.
There is no such thing as a 10 minute job on this EWS stuff. It is a total pain in the "big toe"! TOTAL PAIN. I had one key and a 16 year old kid I didn't trust to actually not lose the thing. I was paranoid. Now I've got extra keys and breathing easier. But for you, I'd suggest you punt and let the dealer mess with this stuff. You'll exceed the cost of the car quickly if you are providing the labor.